I had to do three things to make this work on Mac OSX.
1. Make sure HL2 is installed via Steam
2. Edit the The Stanley Parable.app/Contents/MacOS/run.sh file and change line 8 from
STEAMUSERID=""
to
STEAMUSERID="common"
3. Replace the gameinfo.txt file with the one found here: Pastebin.com
The new gameinfo.txt file was the result of applying this hack: Steamcommunity.com
To start the game, you will need to double-click the "The Stanley Parable" icon in Finder. Running it from Steam will not work (it will complain about not having the Source SDK Base 2007)
stevearc
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